The present invention relates to a process for the preparation of isoalkenes and other useful hydrocarbons by catalytic conversion of n-alkanes, bimetallic catalysts which are suitable for this conversion, and a process for the preparation of such catalysts.
This invention further relates to dehydroisomerization of alkanes.
n-butane is presently a major component of gasoline. It is relatively cheap and plentiful and offers a high octane blending value. However, its Reid vapor pressure (RVP) is very high. Recently, the Environmental Protection Agency (EPA) issued a new proposal to reduce the permitted gasoline RVP from 11.5 psi to 10.5 psi in 1989 and further to 9.0 psi in 1992 as one way to cut hydrocarbon emissions and to lower ozone levels in the atmosphere. These new regulations will prevent refineries from using the highly volatile n-butane as a blending stock during the hot summer season. It is estimated that every one psi reduction in permitted gasoline vapor pressure will lead to a seasonal surplus of 80,000-130,000 bbl/day of n-butane. Therefore, how to utilize the surplus n-butane and to convert it to more valuable products are major concerns for most refineries and n-butane suppliers at this time.
MTBE (methyl tertiary butyl ether) is an effective octane booster. It is made from isobutylene and methanol. The present sources of isobutylene for MTBE production are mainly from by products of steam and catalytic crackers. However, these supplies are limited. Other possible sources are by isomerization of n-butenes taken from steam or catalytic crackers and by dehydrogenation of isobutane taken from field butanes or produced by isomerization of n-butane.
Olefin isomerization processes can be directed towards either skeletal isomerization or double bond isomerization. Skeletal isomerization is concerned with reorientation of the molecular structure in respect to the formation or elimination of side chains. Double bond isomerization is concerned with relocation of the double bond between carbon atoms while maintaining the backbone of the carbon structure. Most isomerization processes give rise only to double bond isomerization.
The minimum Bronsted Acid strengths (and equivalents in H.sub.2 SO.sub.4) required for various acid-catalyzed conversions of hydrocarbons are indicated in the table below.
______________________________________ Minimum Bronsted Acid Strength Required For The Acid-Catalyzed Conversions of Hydrocarbons H.sub.R Required Reaction Type ______________________________________ &lt; + 0.8 Cis-trans Isomerization 1.2 wt % H.sub.2 SO.sub.4 of Olefins &lt; - 6.6 Double-bond Migration 48 wt % H.sub.2 SO.sub.4 &lt; - 11.6 Skeletal Isomerization 68 wt % H.sub.2 SO.sub.4 &lt; - 16.0 Cracking of Alkanes 88 wt % H.sub.2 SO.sub.4 ______________________________________
It is frequently necessary to convert olefins into other olefins having a different skeletal arrangement. For example, normal butenes are converted into isobutene for polymerization, alkylation, disproportionation, etc. Similarly, normal amylenes must be converted to isoamylenes prior to dehydrogenation to isoprene.
While a number of catalytic materials possess some activity for such a conversion, not all possess sufficient selectivity to be economical. Because the feeds are generally the relatively reactive olefins, many catalysts cause undesirable side reactions such as polymerization or cracking. Consequently, there is a continuing interest in the development of new skeletal isomerization catalysts and processes for isomerizing alkanes as well as alkenes to improve efficiencies and to give optimum results for various industrial requirements. A comprehensive review is provided by V. R. Choudhary in "Catalytic Isomerization of n-butene to Isobutene," Chem. Ind. Dev, pp. 32-41 (1974).
It is generally known that n-paraffins with, for example, 4 to 7 carbon atoms can be converted to the corresponding isomeric paraffins by using suitable acid catalysts in the temperature of from 100.degree. to 250.degree. C. Examples of this process are the numerous isomerization processes used in the petrochemical and mineral oil industries for increasing the octane number of light, paraffinic mineral oil fractions. Furthermore, it is known that, in contrast to this, olefins of the same number of carbon atoms cannot be converted to the corresponding isoolefins except under difficult conditions, for example at very high temperatures and with poor yield. The attempts hitherto described in the literature for the direct isomerization of the skeleton of e.g. n-butene to give isobutene or e.g. of n-pentene to give isopentenes over catalysts arranged in a fixed bed are characterized by only initially high yields and selectivities, which diminish and deteriorate considerably after a short period of operation, often after only a few hours. The deterioration in the yields and selectivities is generally attributed to the loss of actively effective catalyst surface or to the loss of active centers. In addition to this, high coking rates, formation of oligomers and cracking reactions are observed.
As is known, butylenes or butenes exist in four isomers: butene-1, cis-butene-2, its stereo-isomer transbutene-2, and isobutene. Conversions between the butenes-2 are known as geometric isomerization, whereas those between butene-1 and the butenes-2 are known variously as position isomerization, double-bond migration, or hydrogen-shift isomerization. These three isomers are not branched and are known collectively as normal or n-butenes. Conversion of the n-butenes to isobutene, which is a branched isomer, is widely known as skeletal isomerization. The same general terminology is used when discussing skeletal isomerization of other n-alkanes and n-alkenes.
Isobutene has become more and more important recently as one of the main raw materials used in the production of methyl tert-butyl ether (MTBE), an environmentally-approved octane booster to which more and more refiners are turning as metallic additives are phased out of gasoline production. However, processes for the skeletal isomerization of olefins e.g., to produce isobutene, are relatively non-selective, inefficient, and short-lived because of the unsaturated nature of these compounds. On the other hand, positional and skeletal isomerization of paraffins and alkyl aromatics are fairly well established processes, in general utilizing catalysts typically comprising metallic components and acidic components, under substantial hydrogen pressure. Since paraffins and aromatics are stable compounds, these processes are quite successful. The heavier the compounds, in fact, the less severe the operating requirements. Olefins, however, are relatively unstable compounds. Under hydrogen pressure, they are readily saturated to the paraffinic state.
Furthermore, in the presence of acidity, olefins can polymerize, crack and/or transfer hydrogen. Extensive polymerization would result in poor yields, and short operating cycles. Similarly, cracking would reduce yield. Hydrogen transfer would result in saturated and highly unsaturated compounds, the latter being the common precursors for gum and coke. Any theoretical one step process for producing skeletal isomers of, for example, n-butenes, would have to be concerned with the unwanted production of butanes and the reverse problem of production of butadienes. In addition to these problems, it is well known that skeletal isomerization becomes more difficult as hydrocarbons get lighter.
Skeletal isomerization of olefins is known to be accomplished by contacting unbranched or lightly branched olefins with acidic catalysts at elevated temperatures. The process is generally applicable to the isomerization of olefins having from 4 to about 20 carbon atoms and is especially applicable to olefins having from 4 to about 10 carbon atoms per molecule. The process may be used to form isobutene from normal butenes, methyl pentenes and dimethyl butenes from normal hexenes, and so forth.
Known skeletal isomerization catalysts include aluminas and halogenated aluminas, particularly F- or Cl-promoted aluminas. Supports employed in such catalysts are either alumina or predominantly alumina due mainly to the high acidity of alumina. See Choudhary, V. R., "Fluorine Promoted Catalysts: Activity and Surface Properties", Ind. Eng. Chem., Prod. Res. Dev., 16(1), pp. 12-22 (1977) and U.S. Pat. No. 4,400,574. Numerous catalysts employ a metal or metal oxide in conjunction with a halide-treated metal oxide. For example, U.S. Pat. No. 4,410,753 discloses isomerization catalysts comprising Bi.sub.2 O.sub.3 on fluorided alumina and U.S. Pat. No. 4,433,191 discloses skeletal isomerization catalysts comprising a Group VIII metal on halided alumina. Many of the catalysts including halide-treated components require periodic addition of halide materials to maintain catalyst activity; for example, see U.S. Pat. Nos. 3,558,734 and 3,730,958. An average yield for isobutene of 25 weight percent (within an observed range of 17 to 33 percent) is typically reported when using halided catalysts, based upon a review of various patents cited in this disclosure.
Various techniques have been employed to improve the effectiveness of materials such as alumina and silica as structural isomerization catalysts. For example, U.S. Pat. No. 3,558,733 discloses methods for activating alumina catalysts with steam, U.S. Pat. No. 4,405,500 discloses catalysts prepared by controlled deposition of silica on alumina and U.S. Pat. No. 4,587,375 discloses a steam-activated silicalite catalyst. In addition, various metal oxides have been used to improve the effectiveness of catalysts based upon alumina, silica or the like.
Zeolitic materials, especially in their hydrogen forms, are known to behave as strong acids. Due to their narrow yet regular pore size they are quite effective in catalyzing olefin polymerization. Unfortunately the pores are soon plugged due to deposition of polymeric materials and frequent catalyst regeneration is necessary to maintain activity.
Therefore, among the objects of this invention are improved processes for the dehydroisomerization of n-alkanes and olefins, especially for the dehydroisomerization of n-butanes to form isobutene. A more specific object is an easily prepared, stable, active multifunctional isomerization catalyst and processes for the skeletal isomerization of hydrocarbon species including n-alkanes and olefins. Other objects and advantages of the invention will be apparent from the following description, including the drawing and the appended claims.